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Norntates PUBLISHED by the AMERICAN MUSEUM of NATURAL HISTORY CENTRAL PARK WEST at 79TH STREET, NEW YORK, N.Y AMERICANt MUSEUM Norntates PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, N.Y. 10024 Number 3187, 43 pp., 7 figures December 31, 1996 Middle-Ear Ossicles of the Multituberculate Kryptobaatar from the Mongolian Late Cretaceous: Implications for Mammaliamorph Relationships and the Evolution of the Auditory Apparatus* GUILLERMO W. ROUGIER,' 2 JOHN R. WIBLE,3 AND MICHAEL J. NOVACEK4 ABSTRACT Partial malleus, ectotympanic, and stylohyal After comparison of these elements with those along with a fragment of a possible stapes are of other mammaliamorphs, a matrix of 62 char- described for the multituberculate Kryptobaatar acters of the ossicles and basicranium across 19 dashzevegi from the Late Cretaceous of Mongo- taxa was compiled and analyzed with PAUP Four lia. The malleus, represented by the anterior pro- equally most parsimonious trees were obtained, cess (composed of the goniale), is attached and which differ in the relationships of multitubercu- only partially delimited by sutures from a segment lates, monotremes, and triconodonts with regard of the anterior crus of the ectotympanic. The pos- to the prototribosphenidan lineage. Monotremata sible stapes fragment may be part of a crus of a is either the sister group of Theriiformes (Multi- stirrup-shaped bone. tuberculata + Theria) or united with Multituber- * This is contribution number 21 of the Mongolian Academy of Sciences-American Museum of Natural History Paleontological Project. I Frick Research Fellow, Department of Vertebrate Paleontology, American Museum of Natural History. New York, NY 10024. 2 Museo Argentino de Ciencias Naturales, CONICET, Buenos Aires, Argentina. 3Research Associate, Department of Mammalogy, American Museum of Natural History; Department of Anatom- ical Sciences and Neurobiology, School of Medicine, University of Louisville, Louisville KY 40292. 4Curator, Department of Vertebrate Paleontology, American Museum of Natural History. Copyright X) American Museum of Natural History 1996 ISSN 0003-0082 / Price $3.80 2 AMERICAN MUSEUM NOVITATES NO. 3187 culata in a monophyletic grouping. Because the distic analysis suggests that definitive middle-ear ossicles are poorly or wholly unknown for most ossicles evolved in the last common ancestor of of the taxa considered, their phylogenetic bearing multituberculates, monotremes, triconodonts, and is limited. trechnotherians (symmetrodonts + therians) and The phylogenetic information derived from the that the triangular molar cusp pattern of kueh- transformations of the postdentary bones/ear os- neotheriids and trechnotherians is likely conver- sicles and the dentition is not congruent. Our cla- gent. INTRODUCTION The Reichert-Gaupp Theory (Reichert, middle-ear ossicles is suggested in the het- 1837; Gaupp, 1913) summarizes our under- erogeneous origin of the involved elements, standing of the homologies of the postdentary which include dermal ossifications (the ec- bones of non-mammalian amniotes with the totympanic and the anterior process of the middle-ear bones of mammals (for modem re- malleus) and derivatives from the mandibular views see Moore, 1981; Starck, 1979; and No- or first arch cartilage (the articular portion of vacek, 1993). The transformation of lower jaw the malleus and the incus), the hyoid or sec- elements into auditory structures, one of the ond arch cartilage (most of the stapes), and hallmarks of Mammalia (sensu Rowe, 1988, the auditory capsule (part of the stapedial the clade including the common ancestor of footplate) (Gegenbaur, 1898; Gaupp, 1908, extant mammals and all its descendants), had 1913; Anson et al., 1960). This complex pro- far-reaching repercussions for the origin and vides a wealth of anatomical variation that is subsequent evolution of the mammalian skull. widely accepted to be of significance for sys- Regions of the head thought to have been no- tematic and functional analyses (Fleischer, tably modified include the basicranium, the lat- 1973, 1978; Novacek and Wyss, 1986; Gau- eral wall of the braincase, and the masticatory din et al., 1996). Yet, study of the early evo- apparatus (Allin, 1975; Crompton and Hylan- lution of this system in the fossil record has der, 1986; Maier, 1987, 1993; Kuhn and Zeller, been hampered by the exceedingly fragile 1987; Allin and Hopson, 1992). nature of the middle-ear bones, which are The formation of an ossicular chain trans- only loosely attached to the skull base. A mitting sound from the outer to the inner ear bony bulla, which as a rule encloses the mid- from former load-bearing elements of the dle ear and protects the ossicles in living lower jaw can be traced in the fossil record therian mammals, is lacking in most early as well as in the ontogeny of living mammals mammals. Consequently, middle-ear bones (Reichert, 1837; Gaupp, 1913; Allin, 1975; are almost never preserved in Mesozoic Maier, 1990). The bony elements involved in mammals, even in fully articulated speci- mammals (and their homologs in non-mam- mens. The only ear ossicles reported from malian amniotes) are the ectotympanic (an- Mesozoic mammals are the stapes, malleus, gular), malleus (articular and prearticular or and incus of the multituberculate Chulsan- goniale), incus (quadrate), and finally, the baatar (Hurum et al., 1995) and the stapes stapes (columella auris or stapes). To date, of an indeterminate placental (Archibald, our understanding of the fate of the postden- 1979); both are Late Cretaceous in age. tary bones represents one of the most suc- Basal Mammaliaformes (sensu Rowe, cessful interplays between paleontology and 1988, the clade including the common an- embryology in establishing the major fea- cestor of Morganucodon and Mammalia plus tures of a complex evolutionary event. Nev- all its descendants) ought to play a prominent ertheless, aspects of this transformation are role in the elucidation of competing hypoth- still questioned, and radically new hypothe- eses about the origin of the mammalian au- ses (Jarvik, 1980) departing from the classic ditory apparatus, because theoretically they Reichert-Gaupp Theory and its modem for- can provide characters and combination of mulation (Allin, 1975, 1986; Allin and Hop- characters not present among living taxa son, 1992) have been proposed. (Gauthier et al., 1988; Donoghue et al., The complex history of the mammalian 1989). Fossils also can provide the historical 1996 ROUGIER ET AL.: KRYPTOBAATAR AND MAMMALIAMORPH RELATIONSHIPS 3 succession of events, permitting determina- and the number has increased since that re- tion of minimal ages for character transfor- port was submitted. As of that report, mul- mations (Novacek, 1992c; Norell, 1992; No- tituberculates represented over 80% of all rell and Novacek, 1992; Smith, 1994). Un- mammal skulls and skeletons collected from fortunately, the available information on this extremely rich locality. middle-ear bones in Mesozoic mammals is All Asian Late Cretaceous multitubercu- so deficient that these elements have played lates have been assigned to Taeniolabidoidea. an almost negligible role in our understand- Gobibaatar parvus, originally described as a ing of early mammal relationships. A few ptilodontoid (Kielan-Jaworowska, 1970), is characters, such as postdentary bones at- now regarded as a junior synonym of Kryp- tached to lower jaw, or suspended from cra- tobaatar (Kielan-Jaworowska, 1980). At nium, have been used for phylogenetic pur- present, eight monospecific genera of multi- poses (Rowe, 1988; Wible, 1991). However, tuberculates are recognized from the Mon- characters so general do not adequately re- golian Late Cretaceous. However, Tugrig- flect the numerous character transformations baatar saichaenensis, known only from the that ought to be expressed in a phylogenetic type specimen, an incomplete skull and low- analysis. Better preserved material and de- er jaws (Kielan-Jaworowska and Dashzeveg, tailed descriptions are needed so that these 1978), is likely a junior synonym of Kryp- elements can be included in the reconstruc- tobaatar (Rougier, Novacek, and Dashzeveg, tion of the overall framework of mammalian in prep.). evolution. Most of the multituberculates collected We provide here the' first substantial evi- from the Gobi Desert by the Mongolian- dence of the malleus and ectotympanic in American Expedition, including the two Mesozoic multituberculates, derived from specimens described here, are referable to Kryptobaatar dashzevegi from the Late Cre- Kryptobaatar dashzevegi (Kielan-Jaworow- taceous of Mongolia. Our description of ska, 1970), the most abundant mammalian these elements furnishes us the opportunity taxon from the to compare them with the ear ossicles pre- Djadokhta Formation (Kie- viously described in other multituberculates, lan-Jaworowska, 1974). The first of these i.e., the Late Cretaceous Chulsanbaatar (Hu- specimens, cataloged as PSS-MAE-1 13 (fig. rum et al., 1995) and the apomorphic Paleo- 1) in the Geological Institute, Ulaan Baatar, cene taeniolabidid Lambdopsalis (Miao and was found in Tugrugeen Shireh, also called Lillegraven, 1986; Miao, 1988; Meng, 1992; Toogreeg. This well-known locality (Kielan- Meng and Wyss, 1995), and in living mam- Jaworowska and Dashzeveg, 1978) has mals. In addition to the fragmentary malleus yielded numerous mammal and dinosaur re- and ectotympanic, we also describe a prob- mains, including the famous fighting dino- able fragment of stapes and the stylohyal in saurs (Kielan-Jaworowska and Barsbold, Kryptobaatar. Finally, a discussion
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